The invention relates generally to electronic acquisition of fingerprints and, more particularly, to a system and a method of protecting a platen of an optical fingerprint imaging device.
Electronic fingerprint acquisition systems have been introduced in recent times to eliminate the need for ink or a chemical reagent. In such systems, the subject's finger tip is pressed against or rolled over a planar or curved transparent platen in an optical system. Light is directed through the platen and is reflected from the finger tip into an optical path including mirrors or prisms, a lens, and an electronic image sensor such as a CCD imager of the type employed in video cameras. The image of the fingerprint may be digitized, analyzed, printed out, stored, transmitted, or compared with an existing fingerprint or fingerprints.
Fingerprint identification is an exacting science that requires the comparison of the many minute and unique characteristics of each fingerprint, including the pattern of ridge endings and ridge bifurcations and locations and directions of these features, known in the art as minutiae, of each person's fingerprint. The total collection of minutiae should be complete enough to collectively comprise a signature or template uniquely identifying the owner of the fingerprint. Comparisons between the template of an unknown fingerprint and templates of fingerprints of known persons may be accomplished manually or by an electronic system. The process of matching fingerprint templates to identify an unknown fingerprint requires the correlation of a minimum number of minutiae for accuracy. Regardless of the image capture method and regardless of whether the fingerprints are interpreted by a trained expert or by a machine, the clarity and resolution of an acquired fingerprint image is obviously of paramount importance. Without enough clarity, enough minutiae may not be obtained to make a clear match, if a match can be made at all.
In optical imaging devices, a finger tip is placed on a platen and a fingerprint image of that finger tip is derived from the selective reflection of light from the platen. Reflected light representative of fingerprint minutiae is converted into data by the imaging sensor, and that data is assembled by the imaging device into a fingerprint image. If assembled successfully, the fingerprint image has been “acquired.” However, if a fingerprint image of low clarity and resolution is produced because of a cloudy, distorted, scratched, or imperfect platen, the number of minutiae regarding that fingerprint may be so limited that correlation of that minutiae with a data base of fingerprint images may result in no correlation with another fingerprint or may result in a correlation with a large number of fingerprints thus making it impossible to identify this unknown fingerprint.
Unclear fingerprint images in electronic acquisition systems may be caused by (1) lack of contrast between the ridges and valleys, (2) interference caused by latent fingerprints on the platen, (3) dirt on the platen resulting from use or environmental conditions, (4) scratches or other defects to the platen resulting from use, vandalism, or environmental conditions, (5) excessive pressure between the finger tip and the platen, (6) poor contact between the finger tip and platen resulting in a poor reflected image, (7) distortion caused by rolling the finger over the platen, (8) poor resolution of the optical equipment, and (9) gross distortion resulting from system configuration. Other conditions may also result in an unclear image. Whatever the cause, an unclear fingerprint cannot be as accurately read or compared with an existing fingerprint for verification or identification purposes.
A large variety of electronic fingerprint imaging devices have been proposed in the art for improving the fingerprint capturing process. What many prior fingerprint imaging devices have in common is a relatively costly platen that is integrated into the device. As shown in FIGS. 1 and 2, the platen 20 of some prior fingerprint imaging devices 21 have an optically transparent or clear rigid first layer 24 covered by a second layer 26 made of a tacky polymer for improving the quality of a fingerprint image. In this case, the first layer is raised slightly above the housing 28 of the imaging device, and the first layer forms a raised wall around its periphery to form a pocket 30 that receives the second layer. The second layer may also be adapted to improve optical contact with a dry finger tip.
As shown in FIG. 3, a fingerprint image of a finger tip 32 is produced when some incident light 34 from a light source (not shown) passes through the first and second layers 24 and 26 and is reflected at portions of the second layer beneath valleys 36 on the finger tip due to a difference in refractive index between the second layer and air above the second layer. The reflected light 38 associated with the valleys of the finger tip is received by an electronic image sensor (not shown). Incident light is generally absorbed at portions of the second layer in contact with ridges of the finger tip. However, some of the incident light may also be reflected at portions of the second layer in contact with ridges of the finger tip, which results in reduced fingerprint image contrast.
To improve the image contrast, the second layer 26 of the platen 20 may also be designed to deform under pressure from the ridges 40 of the finger tip 32 pressed against it. The ridges and corresponding deformations are exaggerated for purposes of illustration. The deformations under the ridges cause the incident light 42 that is reflected beneath the finger tip ridges to disperse, which desirably reduces the amount of reflected light 44 associated with the ridges from entering the electronic image sensor, thereby making it easier for the imaging device to distinguish between ridges and valleys. Although not intending to be bound by theory, the above discussion and referenced drawings are provided for purposes of illustration only.
The large variety of electronic fingerprint imaging devices all have a common problem—platen maintenance. It is a completely natural and unavoidable occurrence that oils and dirt accumulate on the platen 20 during its use. Environmental conditions such as blowing dust and ultraviolet light can also contribute to a build-up of dirt, grease, or damage on the surface of the platen that obscures fingerprint images. Attempts to clean a dirty platen with a tissue may permanently abrade the surface of the platen and further obscure fingerprint images.
The platen 20 may degrade more rapidly in certain environments. Healthcare facilities often employ fingerprint imaging devices on medication dispensing cabinets, storage cabinets, and other equipment. Unfortunately, many platens are degraded by antibacterial and petroleum-based moisturizing lotions that are used on the hands of healthcare personnel and by antiseptic cleaning agents used on equipment. The tacky polymer second layer 26 of some platens has been found to chemically break down from such hand lotions and cleaning agents used in healthcare facilities. Chemical breakdown of the second layer 26 makes the second layer less resistant to scratching and pitting thus becoming more abraded with normal use.
When there are scratches, pits, or other defects 46 on an exterior surface 48 of the platen 20, as shown in FIG. 4, the incident light 34 that is reflected beneath the finger tip valleys 36 (FIG. 3) is dispersed and will not provide as clear an image. As such, defects on the surface of the platen undesirably reduce the amount of reflected light 38 associated with the valleys entering the electronic image sensor, making it difficult or impossible for the imaging device to accurately make out minutiae on the fingerprint. When this occurs with some imaging devices, the entire device 21 including the light source, prism, and sensor must be replaced. Other imaging devices allow for the platen alone to be replaced periodically. In either case, such replacements are costly and time consuming and may require special training and tools.
In addition to the inconvenience and cost of replacement, scratches, pits, or other defects 46 on the surface of the platen 20 can have a detrimental impact on workflow. For instance, healthcare professionals often must identify themselves when accessing electronically controlled medication dispensing cabinets, such as the Pyxis MedStation from Cardinal Health (San Diego, Calif.), which are used to store medications and supplies at nursing stations and other convenient locations within a healthcare facility. Fingerprint imaging devices are employed to automate the identification process and to increase security, especially when narcotics and other controlled substances and controlled items are stored in a medication cabinet. A platen 20 that has become cloudy as a result of chemical breakdown and micro abrasions (scratches) on its exterior surface and has become unusable or only intermittently usable to match fingerprints can disable the automatic log in and security features of the medication cabinet, thereby reducing the workflow efficiency in delivering patient care. As used herein, “controlled items” are not limited to narcotics but is a term meant to encompass any item that a healthcare facility desires to control or track.
Hence those skilled in the art have recognized a need for an improved optical fingerprint imaging system and method including a protective device that allows for quick, simple, and less expensive platen maintenance and reduces the need for such maintenance. The present invention satisfies this and other needs.